Semiconductor devices based on Gallium nitride (GaN) can support high voltages and carry a large current. This makes them promising candidates for power semiconductor devices aimed at high power/high-frequency applications. Devices manufactured for such applications exhibit high electron mobility and are referred to as high electron mobility transistors (HEMT), heterojunction field effect transistors (HFET), or modulation doped field effect transistors (MODFET). These types of devices are typically operated at high frequencies (e.g. 100 kHz-10 GHz) and can typically withstand high voltages, e.g., 100 Volts.
GaN HEMT devices typically include an AlGaN barrier layer adjacent to a GaN layer. The difference in the material in these two layers contributes to a conductive two-dimensional electron gas (2 DEG) region near the junction of the two layers and in the layer with the smaller band gap. This 2 DEG allows charge to flow through the device. This feature makes such devices depletion mode devices that are normally “on” when no bias is applied at the gate. Enhancement mode (e-mode, normally “off”) devices, however, typically more desirable as they are safer and easier to control. An enhancement mode device requires a positive bias applied at the gate in order to conduct current.
For a GaN HEMT device to work as an enhancement mode device, however, the 2 DEG region must be depleted.
One of the solutions to achieve normally-off (e-mode) operation for GaN HEMT transistors is to employ a p-GaN layer, i.e. a positively doped GaN layer, in the gate region. The Fermi level in the p-GaN will be pulled towards the valence band, lifting up the band-diagram in the GaN channel region achieving the required e-mode operation. The disadvantage of this solution is that the p-GaN gate is a junction type of gate and can thus cause high gate leakage. A solution to lower the gate leakage is to employ a gate metal, which can form a Schottky barrier towards the p-GaN. In the cross section, shown in FIG. 1, the metal in the Schottky barrier is TiN. However, the leakage of the metal in the Schottky barrier is in large part determined by the active Mg concentration in the p-GaN layer, where a higher concentration leads to a higher gate leakage. This causes a trade-off between the threshold voltage and gate leakage because to achieve a high threshold voltage, a high active Mg concentration is generally needed. However, high active Mg concentration also leads to increased leakage.
US2010/0258841 relates to enhancement mode GaN transistors and deals with the problem of high gate leakage when the transistor is turned on by applying a positive voltage to the gate contact. US2010/0258841 remarks that during growth of the p-GaN layer, Mg atoms diffuse to the growth surface. Hence, when growth is terminated, a heavily doped layer exists at the surface and when a positive bias is applied to the gate contact, a large current is generated due to the high doping at the top of this layer. US2010/0258841 proposes to solve this current leakage problem by reducing the Mg concentration to about 1016 atoms per cm3 near the gate contact, thereby allowing the formation of a Schottky barrier between the gate contact and the p-type GaN. In addition to reducing the Mg concentration near the gate contact, US2010/0258841 proposes to n-dope the p-GaN gate layer by adding Si atoms near the gate contact in order to further reduce the hole density. However, US2010/0258841 does not propose an experimental way enabling the reduction of the Mg concentration near the gate contact. In fact, it can be very difficult to achieve such a reduction because, due to the dynamics of the MOCVD growth, a surface peak Mg concentration will typically be present near the top. Furthermore, Si counter-doping is not a solution since it requires matching exactly the Mg concentration and requires, therefore, full counter doping. Indeed, if the counter-doping is incomplete, gate leakage is to be expected and if the counter-doping is excessive, it will create highly doped n-layer near the top of the device.
There is, therefore, a need in the art for new ways to deal with the problem of high gate leakage.